The present invention relates to a method for the decolorizing purification of a halogenated silane compound or, more particularly, to a method for the decolorizing purification of a liquid halogenated silane compound such as methyl trichlorosilane, vinyl trichlorosilane and the like useful, for example, as a starting material in the synthesis of an oxime group-containing organosilane compound.
Halogenated silane compounds in general are useful as a starting material for the synthetic preparation of various kinds of organosilicon compounds or, in particular, silicone compounds. A serious problem in the use of a halogenated silane compound as a starting material organosilicon compounds is that the halogenated silane compound is sometimes colored due to the trace content of various kinds of coloring heavy metals so as to greatly decrease the commercial value of the organosilicon compound as the product therefrom due to unavoidable coloration. This problem is particularly important when the silicone product produced from the halogenated silane compound is used in an application to colorless or white materials such as fibrous materials or in the preparation of silicone rubbers of transparency formulation. In this regard, it is eagerly desired to develop a simple and reliable method for the decolorizing purification of an organochlorosilane used as a starting material for an oxime group-containing organosilicon compound. Colored silane compounds can of course be decolorized and purified by distillation but this procedure is relatively expensive resulting in an increase of the production cost. Further, the coloring impurities in a halogenated silane compound can be removed by using active carbon but the efficiency of this method is relatively low and far from satisfactory.
Oximesilane compounds are prepared usually by the reaction of an organochlorosilane such as methyl trichlorosilane with a stoichiometric amount of an oxime compound such as methyl ethyl ketoxime in the presence of a stoichiometric amount of an organic base as an acceptor of hydrogen chloride formed as a by-product of the dehydrochlorination reaction as is disclosed in Japanese Patent Publication 39-29837. On the other hand, Japanese Patent Publication 1-21834 discloses a method in which the reaction of an organochlorosilane and an oxime compound is conducted by using the oxime compound in an amount of twice of the stoichiometric amount so that the excess of the oxime compound serves as the acid acceptor. Alternatively, Japanese Patent Kokai 63-227592 discloses a preparation method of an oximesilane compound in which an organochlorosilane compound is reacted with a stoichiometric amount of an oxime compound under continuous blowing of ammonia gas into the reaction mixture in an amount of 1.04 to 1.46 times of the stoichiometric amount to serve as the acid acceptor.
The oximesilane compound prepared by the above described method is always more or less colored in brown or green due to the presence of a few ppm of a compound of heavy metals such as iron, chromium, nickel, manganese and the like in the form of FeCl.sub.3, CrCl.sub.3, NiCl.sub.2 and MnCl.sub.2. Complete prevention of coloration of an oximesilane compound thereby is a very difficult matter from the economical standpoint despite the countermeasures thus far undertaken including purification of the starting materials, appropriate selection of the material of the reaction vessels and so on in so far as the halogenated silane compound as the starting material thereof is contaminated with a few ppm of these heavy metal compounds.
Similar difficulties are encountered relative to halogenated silane compounds due to contamination with heavy metals to cause coloration of the silane compound in yellow to brown. Even when a halogenated silane compound is purified by distillation to completely remove heavy metal impurities, such a purified and colorless halogenated silane compound is already contaminated when it is used as a starting material in the next step because of the many chances by which a heavy metal impurity is taken into the silane compound by contacting with pipe lines, storage vessels and the like.